In an electroerosion machine, for the sake of economy it is customary that a used machining liquid medium from an electroerosion zone is treated and regenerated for recycling into the electroerosion zone. It is desirable that the recycled liquid medium be not only free from eroded particles and sludges but hold a prescribed electrical property that can be measured in terms of specific resistance or conductance. A particular electrical property of the flushing liquid medium need be substantially maintained during a given electroerosion machining operation to obtain optimum machining results. The flushing liquid is typically an aqueous medium or a liquid hydrocarbon having a predetermined resistivity or conductivity. Thus, a system has been known which includes means that is connected for being selectively brought into operation to regenerate the degraded flushing medium when the conductivity thereof determined by a conductivity tester exceeds a predetermined value (see U.S. Pat. Nos. 3,417,006 and 3,928,163). In that system, the regeneration means is designed to remain connected and active continuously for the liquid medium until its conductivity is indicated to fall below the same or another, lower predetermined value.
It has been found that such a system is inefficient and also does not operate in a manner to minimize deviations from a desired level or range of the conductivity of the renewed fluid serving for electroerosion. Due to an inherent delay in the circulation lines after a sharp change occurs in the electroerosion zone until a command to rectify the fluid conductivity is actually reflected on the renewed fluid delivered into the same zone, there tends to develop excessive variations, both upwards and downwards, of the conductivity of the fluid recycled into the electroerosion zone. As a result, the system would repeat "hunting" making unavoidable unnecessary and even detrimental excessive rises and drops of the conductivity and such changes in an electrical property of the fluid in the electroerosion zone.